There are generally, three types of golf balls. The first type is a wound ball wherein a vulcanized rubber thread is wound under tension around a solid or semi-solid core, and thereafter is enclosed in a single or multi-layer covering of tough, protective material.
A second type of golf ball is a one-piece ball formed from a solid mass of moldable resilient material which has been cured to develop the necessary degree of hardness. One-piece molded balls do not have an enclosing cover. A third type of ball is a multi-piece (two or more pieces) non-wound ball which includes a solid or liquid core of one or more layers and a cover having one or more layers formed over the core.
Multi-piece non-wound golf balls typically have a cover which contains a substantial quantity of ionomer. Useful ionomers include those sold by E.I. Dupont de Nemours and Company under the name Surlyn® as well as those sold by Exxon under the name Iotek®. Ionomers impart toughness and cut resistance to the covers. It would be useful to develop golf ball covers which contain substantial quantities of non-ionomeric materials and which have the durability, scuff resistance, cut resistance and other playability properties of ionomeric golf ball covers.
Polyurethanes are polymers which are used to form a broad range of products. They are generally formed by mixing two primary ingredients during processing. For the most commonly used polyurethanes, the two primary ingredients are a polyisocyanate (for example, diphenylmethane diisocyanate monomer (“MDI”) and toluene diisocyanate (“TDI”) and their derivatives) and a polyol (for example, a polyester polyol or a polyether polyol).
A wide range of combinations of polyisocyanates and polyols, as well as other ingredients, are available. Furthermore, the end-use properties of polyurethanes can be controlled by the type of polyurethane utilized, i.e., whether the material is thermoset (crosslinked molecular structure) or thermoplastic (linear molecular structure).
Crosslinking occurs between the isocyanate groups (—NCO) and the polyol's hydroxyl end-groups (—OH). Additionally, the end-use characteristics of polyurethanes can also be controlled by different types of reactive chemicals and processing parameters. For example, catalysts are utilized to control polymerization rates. Depending upon the processing method, reaction rates can be very quick (as in the case for some reaction injection molding systems—“RIM”) or may be on the order of several hours or longer (as in several coating systems). Consequently, a great variety of polyurethanes are suitable for different end-uses.
Polyurethane has been used for golf balls and other game balls as a cover material. Commercially available polyurethane golf balls have been made of thermoset polyurethanes. A polyurethane becomes irreversibly “set” when a polyurethane prepolymer is crosslinked with a polyfunctional curing agent, such as polyamine and polyol. An isocyanate that is reacted with a polyamine forms a polyurea. The term “polyurethane” is often used to describe polyurethane/polyurea systems. The prepolymer typically is made from polyether or polyester. Diisocyanate polyethers are preferred because of their water resistance.
The physical properties of thermoset polyurethanes are controlled substantially by the degree of crosslinking. Tightly crosslinked polyurethanes are fairly rigid and strong. A lower amount of crosslinking results in materials that are flexible and resilient. Thermoplastic polyurethanes have some crosslinking, but purely by physical means. The crosslinking bonds can be reversibly broken by increasing temperature, as occurs during molding or extrusion. In this regard, thermoplastic polyurethanes can be injection molded, and extruded as sheet and blown film. They can be used to up to about 350° F. and are available in a wide range of hardnesses.
U.S. Pat. No. 5,006,297 indicates that while thermoplastic and thermosetting polyurethanes are known, thermosets have been found to produce better cover stocks for golf balls. Additionally, while thermoplastic polyurethanes can be used to form game balls, they lack the scuff and cut resistance of a crosslinked polyurethane. Similarly, thermoplastic polyurethanes do not readily crosslink.
A further disadvantage of using thermosetting polyurethanes to form game ball covers is that scrap material (i.e. sprues, runners and/or reject parts) and cover stock from off-spec balls cannot be reused without substantial processing. It would be useful to develop a high quality game ball utilizing a polyurethane cover material which is subject to thermal degradation prior to final processing. In such a case, the scraps formed in the cover molding stage could be conveniently re-used to form additional game ball covers. A further advantage would be to produce a polyurethane based game ball which, when molded and then crosslinked, is resistant to thermal degradation. This would produce an improved game ball which could also withstand prolonged exposure to heat during use or storage.